Approximability of the Subset Sum Reconfiguration Problem

The subset sum problem is a well-known NP-complete problem in which we wish to find a packing (subset) of items (integers) into a knapsack with capacity so that the sum of the integers in the packing is at most the capacity of the knapsack and at least a given integer threshold. In this paper, we study the problem of reconfiguring one packing into another packing by moving only one item at a time, while at all times maintaining the feasibility of packings. First we show that this decision problem is strongly NP-hard, and is PSPACE-complete if we are given a conflict graph for the set of items in which each vertex corresponds to an item and each edge represents a pair of items that are not allowed to be packed together into the knapsack. We then study an optimization version of the problem: we wish to maximize the minimum sum among all packings in the reconfiguration. We show that this maximization problem admits a polynomial-time approximation scheme (PTAS), while the problem is APX-hard if we are given a conflict graph

An exact algorithm for the Boolean connectivity problem for k-CNF

We present an exact algorithm for a PSPACE-complete problem, denoted by CONNkSAT, which asks whether the solution space for a given k-CNF formula is connected on the n-dimensional hypercube. The problem is known to be PSPACE-complete for k≥3, and polynomial solvable for k≤2(Gopalan et al., 2009).We show that CONNkSAT for k≥3 is solvable in time O((2−ϵ_{k})[n]) for some constant ϵ_{k}>0, where ϵk depends only on k, but not on n. This result is considered to be interesting due to the following fact shown by Calabro: QBF-3-SAT, which is a typical PSPACE-complete problem, is not solvable in time O((2−ϵ)[n]) for any constant ϵ>0, provided that the SAT problem (with no restriction to the clause length) is not solvable in time O((2−ϵ)[n]) for any constant ϵ>0